The conversion of the common hydrocarbon transportation fuels, such as gasoline, into a hydrogen-rich gas suitable for use in fuel cell systems used to produce electricity has yet to be demonstrated in a practical system. The hydrocarbon processing industry has developed technologies for converting low-value feedstocks to hydrogen and synthesis gas. Common approaches include steam reforming, partial oxidation, and autothermal reforming.
Steam reforming systems are used in over 90% of industrial hydrogen and synthesis gas plants. Feedstock and steam are reacted in heated tubes filled with catalysts (typically nickel based) to convert the hydrocarbons into principally hydrogen and carbon monoxide. Due to the endothermic nature of steam reforming, it is uniquely suited to integration with sources of available high-grade waste heat in many industrial applications. In addition to its high conversion efficiency, steam reforming avoids dilution by nitrogen which is inherent in partial oxidation and autothermal reforming systems. Such diluent gases increase the mass flows and thereby increase the size and cost of necessary equipment. While steam reforming has many desirable benefits, it is adversely affected by sulfur content in the feedstock. Land-based systems thus commonly employ a hydrogenation process followed by sulfur absorption to remove the sulfur prior to steam reforming. Sulfur removal and thermal integration are thus two critical issues facing the use and adaptation of steam reforming processes into a compact, on board fuel processing system for vehicles such as automobiles.
Partial oxidation systems are based on substoichiometric combustion to achieve the temperatures necessary to reform the hydrocarbon feedstock. Decomposition of the feedstock to primarily hydrogen and carbon monoxide occurs through thermal cracking reactions at high temperatures (2200-3000.degree. F.). The heat required to drive the reactions is supplied by burning a fraction of the fuel; therefore, the efficiency of such systems is limited by the amount of fuel burned. In addition, because air is commonly used as the oxidant in small systems, significant quantities of diluent nitrogen must be accommodated, thereby increasing the size of the processing equipment.
Catalysts have been used with partial oxidation systems (catalytic partial oxidation) to promote conversion of various sulfur-free feedstocks, such as ethanol, into synthesis gas. The use of a catalyst can result in acceleration of the reforming reactions and can provide this effect at lower reaction temperatures than those which would otherwise be required in the absence of a catalyst. The desirable result can be soot-free operation, which is a common problem with partial oxidation approaches, and improved conversion efficiency from smaller and lighter weight equipment. However, common catalysts are susceptible to coking by feedstocks which are high in aromatic content (such as automotive fuels) at the low steam-to-carbon ratios employed (typically 0 to 1 mole of H.sub.2 O per mole of carbon in the feedstock).
Autothermal reforming (ATR) is a variation on catalytic partial oxidation in which increased quantities of steam are used to promote steam reforming and reduce coke formation. A significant advantage of ATR technology is direct thermal integration of the heat source (partial combustion) and the catalyst bed. This considerably simplifies start-up procedures and reduces transients for load-changes. The high temperature of the ATR catalyst bed (1800-2200.degree. F.) imparts considerable sulfur tolerance, which is desirable for sulfur-bearing automotive fuels. However, ATR requires either air compression with associated nitrogen dilution of product gas or an on-board source of oxygen, both of which add size and cost to the fuel processing system.
Although none of the fuel processing technologies described above have been demonstrated in a suitably compact, lightweight, and cost-effective configuration for transportation applications, such as their use in electric cars, steam reforming has several unique advantages over the other approaches. Steam reforming does not require oxidation of the fuel, and thus it offers the potential for superior energy efficiency and reduced size compared to either partial oxidation or autothermal reforming.
In addition to the fuel processing step, other processing steps may be necessary to reduce the H.sub.2 S and CO content of the fuel gas to meet fuel cell requirements. Absorbent beds can be utilized to remove H.sub.2 S from the fuel gas. Catalytic systems are used industrially to promote conversion of CO and H.sub.2 O to H.sub.2 and CO.sub.2 by the water-gas shift reaction or to selectively burn CO to CO.sub.2. However, each of these processes requires a heat exchanger to control gas temperature, each adds size and weight to the complete fuel processing system, and control can be complex-especially during conditions of varying flow rates.
One alternative to catalytic gas processing is to employ a hydrogen-permeable membrane material to separate essentially pure hydrogen from the remainder of the fuel gas. Typically, these membranes are made from palladium or palladium alloy films supported by porous ceramic substrates. The palladium has high selectivity as well as high permeability for hydrogen. The thermal limits of such membrane materials and their tolerance to poisoning by sulfur and carbon deposits remain to be demonstrated under real-world fuel processing conditions. The high cost of palladium is also an issue for vehicular systems; however, intensive research is being conducted to reduce the required thickness of the film, which will reduce the amount of palladium needed for a given membrane.
Laboratory testing of membrane-walled reactors for steam reforming natural gas has been reported in the literature, but the present inventors are unaware of any application of this technology to on-board, mobile, fuel processing systems.
It is estimated that there are approximately 675 million or more motor vehicles in use worldwide. Almost all of these vehicles currently use internal combustion engines for motive power. Many attempts to produce a cost effective, yet powerful electric car have been made in recent years.
Fuel cells, which use molecular hydrogen (H.sub.2) gas to produce electricity, are known as an efficient electrical power source. Fuel cells are quieter and cleaner than internal combustion engines. However, providing a continuous flow of nearly pure hydrogen to power such fuel cells has made producing an electric car operating on these devices difficult at best. Several considerations must be made when designing a fuel cell powered system that is capable of economically and efficiently competing with the internal combustion engine as a primary motive power source, and these include adequate reliability, size, weight, and cost. The subject invention addresses each of these important considerations.